Exam 1: Virology I Flashcards
Virus
Nucleic acid surrounded by a protein coat = nucleic capsid
Not living organisms but either have DNA or RNA, not both
Obligate intracellularparasites: cannot produce energy, no metabolism, no reproduction, etc.
Naked viruses or have a nuclear envelop (membrane) from the host
Viruses are unable to make energy, synthesize their own proteins, or replicate their genome independently of host cells
Viral Properties vs. Cell
Nucleic Acid: either DNA or RNA (not both)
Proteins: few (cells have many)
Lipoprotein Membrane: envelope present in some (cells all have membranes)
Ribosomes: absent
Mitochondria: absent
Energy/ATP production: None
Enzymes: none or few
Non Enveloped
dsDNA: papovaviridae, adencoviridae, adenovirus
dsRNA: reoviridae
ssRNA: picornaviridae and calicivirus
ssDNA: parvoviridae
Enveloped
dsDNA: poxviridae, herpesviridae, hepadnaviridiae
ssRNA: paramyxoviridae, orthomyxoviridae, rhabdoviridae, retroviridae, arenaviridae, coronaviridae, bunyaviridae, togavirdae (Rubella), and flaviviridae
Viral Shape, Size, and Envelope
Size of viruses can vary- bacteriophage MS2 size and can reach a size as big as Chlamydia
Compared to E.coli, viruses are small
Piconavirus: have RNA as genome and very tightly packed because small
Largest virus (Poxvirus) is as large as Chlamydia but much smaller than E. coli
Viral Structure: Naked and Enveloped
We differentiate enveloped vs. non enveloped
Bacteriophage: capsid with nucleic acid to be injected into the host bacteria; naked/no envelope; the only thing that enters the cell is the DNA/RNA
Human viruses: they enter their capsid protein and nucleic acid; removal of the protein capsid and then freeing of the nucleic acid for reproduction happen within the host
Glycoprotein membrane: allows the glycoproteins to act as receptors that enable the virus to bind/attach to the host cell when it is going to infect it
Classification of Viruses
Several systems for classification of viruses have been used in the past
Most of these have been based on: viral disease, symptoms, and mode of transmission
In 1966, the International Committee on the nomenclature of Viruses was formed. Criteria for classification: mucleic acid type, composition, and size, virus morphology, serological cross-reactivity of group antigens
Morphology, physicochemical properties, genome properties, virus protein properties, biologic properties, antigenic properties
Morphology of Viruses
Size
Shape
Symmetry
Presence or absence of peplomers, which are virus-encoded glycoproteins inserted onto virus envelope
Presence or absence of envelope (membrane)
Physiochemical Properties
Molecular mass Density pH stability Thermal stability Susceptibility to physical and chemical agents such as ether and detergents
Genome Properties
Nucleic acid: DNA or RNA; ss or ds; linear or circular (linear is most common)
Size and type of genome
Sense: positive or negative
Segments (e.g., influenza)
Special features: 5’-terminal cap, 5’-terminal covalently linked protein, 3’-terminal poly[A] trail
Retroviruses contain RT (reverse transcriptase)
Capping enzymes to modify viral mRNA at their 5’ end by adding 7- methylguanosine
Polyadenylation of the 3’ end of viral mRNA
Virally coded enzymes: protein kinases, phosphatases, endonucleases, and RNAses
Virus Protein Properties
Number Size Functional activities Amino acid sequence Modifications (glycosylation) Functional activities (transcriptase, RT, neuraminidase, fusion activities)
Biologic Properties
Natural host range: mode of transmission, vector relationship, pathogenicity, tissue tropism
Antigenic properties: serologic relationship with the host
Universal System of Virus Taxonomy
Families (morphology, genome structure, and strategies of replication).
Suffix “-viridae”
Genera (subdivision): based on physicochemical, serology, cytopathology, or epidemiology.
Viral Architecture
Three groups: Icosahedral symmetry: has 20 faces (e.g., adenoviruses) Helical symmetry (e.g., orthomyxoviruses) Complex structure (e.g., poxviruses): has two membranes, one on the outside and one on the core with nucleic acid (DNA); outside one has the lateral bodies
Capsid vs. Nucleocapsid vs. Envelope
Capsid: protein shell, or coat. Encloses the nucleic acid genome
Nucleocapsid: protein-nucleic acid complex
Enveloped virus in addition to capsid, there is a membrane that surrounds the whole structure; membrane is biological composed of phospholipids with spikes
Outer Shell/Envelope:
1. Protects genome
2. Aids in the process of entry into the cell
3. Packages enzymes essential for the early
steps of the infectious process
Icosahedral Capsid Symmetry: No Envelope
Papillomaviruses/Papovavirus, Parvovirus, Adenovirus
Two key components:
1. Central core (the genome)
2. Outer shell of protein known as the capsid (made of proteins)
The capsid of each virus is an aggregation of morphologic units known as capsomeres
The core and capsid are known as the nucleocapsid
Icosahedral Capsid Symmetry with Envelope
Rubellavirus/Togavirus, Herpes, and HepB
Nucleocapsid is wrapped in an outer envelope
Between the nucleocapsid and envelope is the matrix protein
The viral envelop is derived from the membrane of the host cell
The viral envelope proteins generally appear as spikes known as peplomers made of glycoproteins
Envelope can help with cell attachment, but also a vaccine target
Helical Capsid Symmetry & Envelope
Influenzavirus
The capsid proteins are aggregated around the viral genome as a flexible hollow tube
The coiled nucleocapsid is wrapped in an envelope to form a virion
Virus With a Complex Symmetry
Smallpox/Poxvirus
The structure is much more complex
Features: an external envelope, complex layer of tubular structure, an internal structure made of a DNA-containing core and lateral bodies
Lateral bodies contain various enzymes essential for virus replication
Viruses with this morphology are among the largest of all viruses
DNA Viruses (6): Naked vs. Enveloped
Hepatitis B: enveloped Herpesviruses: enveloped Poxviruses: enveloped Papovaviruses: naked Parvoviruses: naked Adenoviruses: naked
RNA Viruses (5): Naked vs. Enveloped
Picornaviruses: naked Caliciviruses: naked Arenaviruses: enveloped Orthomyxoviruses: enveloped, segmented genome, 8 ss linear fragments Paramyxoviruses: enveloped
RNA to DNA Viruses
Retroviruses (ssRNA)
Hepadnaviruses (dsDNA)
Segmented Genome
Viral genome: fragmented into two or more nucleic acid molecules; all fragments must be present in order to cause infection
Isocapsidic virus: all fragments of segmented genome located in the same virion like the influenza virus
Heterocapsidic virus: each fragment packaged in a different virion, and successful infection requires that at least one RNA of each type enters host cell
example: alfalfa mosaic virus with 4 different RNA segments
Arboviruses (Arthropod-borne)
Not a family of viruses, just all of them are spread by the bites of mosquitoes, ticks, or sandflies
Bunyaviruses
Flaviviruses
Togaviruses
